Immunohistochemical localization of the neuropeptide diazepam binding inhibitor (DBI) and Gamma-amino-buturic acid (GABA) in central and peripheral nervous system was studied. DBI is a newly found polypeptide isolated from rat and human brain. The distribution of DBI in nervous system and also the co-localization of DBI and the major inhibitory neurotransmitter, the GABA, was studied. The results of this immunochemical study show that DBI is located in selective neuronal populations of rat brain. In rat brain DBI immunoreactivity was high in the medial amygdaloid nucleus, many hypothalamic nuclei cortex and in various areas of hippocampus. Whereas low or virtually no DBI immunoreactivity was seen in corpus callosum, internal capsule, optic tract, fornix and granular layer of hippocampus. DBI was found also in peripheral nervous system i.e. in nerve fibers of adrenal medulla and sympathetic ganglia. In the rat cerebral cortex DBI immunoreactivity was located in neurons that are not GABAergic, however, in the cerebellum and in hippocampus DBI might be present also in GABAergic neurons. In primary cultures of hippocampal cells stained with double immunofluorescence technique some cells revealed both DBI and glumatic acid decarboxylase (GAD) staining demonstrating the possiblity of DBI and GABA in the same cells.